1. Field of the Invention
The present invention relates to a method and apparatus for removing minute particles from a surface. More particularly, the invention relates to a method and apparatus for removing minute particles from a surface using thermophoresis to prevent particle redeposition.
2. Background of the Related Art
Particle contamination of surfaces is a concern in many areas of technology. Two areas where such contamination can be a very significant problem are optics, particularly those with critical optical surfaces, and electronic device fabrication. The effect of contaminants on critical optical surfaces (coated or uncoated, dielectric or metal), for example in high power laser optics, can lead to increased optical absorption and a decreased laser damage threshold. As minute particles contaminate optical surfaces, they can serve as sinks for optical power incident on the optical surfaces and thus produce localized heating and possible damage. Large telescope mirrors, and space optics are other applications which require highly decontaminated critical optical surfaces.
In the electronics industry, particle contamination is an important factor in the manufacture of high density integrated circuits. Even in relatively conventional technology using micron or larger circuit patterns, submicron size particle contamination can be a problem. Today the technology is progressing into submicron pattern sizes, and particle contamination is even more of a problem. For device fabrication, particles serve as “killer defects” for only the device that is particle contaminated. The term “device” includes electronic devices, including masks/reticles, optical devices, medical devices, and other devices where particle removal could be advantageous. A particle contaminated mask/reticle prints every device with a defect. At the shorter wavelengths being developed for the next generation of lithography, materials for a protective pellicle for the mask are not available, making particle removal techniques an essential technology in the future. Contaminant particles larger than roughly 10% of the pattern size can create damage, such as pinholes, which interfere with fabrication processes (such as etching, deposition and the like), and defects of that size are a sufficiently significant proportion of the overall pattern size to result in rejected devices and reduced yield. As an example, it has been found that the minimum particle size which must be removed in order to achieve adequate yield in a one Megabit chip (which has a pattern size of one micron) is about 0.1 microns.
Filtration (of air and liquid), particle detection, and contaminant removal are known techniques used in contamination control technology in order to address the problems outlined above. For example, semiconductor fabrication is often conducted in clean rooms in which the air is highly filtered, the rooms are positively pressurized, and the personnel allowed into the room are decontaminated and specially garbed before entry is allowed. In spite of that, the manufactured devices can become contaminated, not only by contaminants carried in the air, but also by contaminants created by the processes used to fabricate the devices.
Removal techniques for contaminants should provide sufficient driving force for removal but without destroying the substrate. Moreover, acceptable removal techniques should provide a minimum level of cleanliness in a reliable fashion. As the particle size decreases, the particle weight becomes less significant as compared to other adhesive forces binding the particle to the surface which it contaminates. Removal of such small particles can potentially damage the substrate.
In general, it has been found that submicron particles are the most difficult to remove. Many of the processes developed to clean integrated circuits, such as ultrasonic agitation, are not effective for micron and submicron particles and indeed, sometimes add contaminants to the substrate.
Laser assisted particle removal has been described in U.S. Pat. No. 4,987,286 issued to Susan D. Allen on Jan. 22, 1991, which is hereby incorporated by reference. U.S. Pat. No. 4,987,286 discloses a method and apparatus for removing minute particles from a surface to which they are adhered using laser technology, and further teaches the use of an energy transfer medium to effect efficient laser assisted particle removal (LAPR). As shown in FIG. 1, a condensed liquid or solid energy transfer medium 23, such as water, is interposed under and around a contaminant particle 22 to be removed from a substrate 20 to which the particle is adhered. Thereafter, the medium 23 is irradiated using laser energy 25 at a wavelength which is strongly absorbed by the medium 23 causing explosive evaporation of the medium 23 with sufficient force to remove the particle 22 from the surface of the substrate.
Another particle removal technique has been to direct the laser energy into the substrate. The laser heated substrate then transfers energy into the energy transfer medium via conduction causing explosive evaporation sufficient to remove the particle from the surface of the substrate. The laser energy can also be directed into the particle(s) to be removed.
Both direct absorption by the energy transfer medium, and substrate and/or particle(s) absorption with subsequent heating of the energy transfer medium can result in efficient LAPR. However, advances in technology have decreased the critical dimensions of various devices, such as, for example, magnetic hard drives, semiconductor devices, masks to make semiconductor devices, etc., and have also increased the surface quality specifications for devices such as large telescope mirrors, space optics, high power laser optics, etc. Therefore, the ability to remove particulate contamination in a noncontact clean fashion becomes ever more important.
One of the challenges of LAPR and other particle removal methods is keeping the particles from redepositing on the surfaces, particularly for very small particles that are not significantly affected by gravity. Several options are available for preventing removed minute particles from redepositing on the cleaned surface. For example, when particles are removed in a vacuum, the mean free path of the particle is long enough to keep it from redepositing and a cooled surface can serve as a particle trap. Also, gas jets parallel to the surface can be used to entrain particles and transport them away from the critical surface.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.